Calculate your Body Mass Index instantly, in metric or imperial units, with your result plotted on a colour gauge, the healthy weight range for your exact height, and how far you are from it. Everything updates live as you type — no button, no sign-up, nothing stored.
How to use the BMI calculator
- Pick metric (cm/kg) or imperial (ft+in/lbs).
- Type your height and weight — the result appears immediately.
- Read your BMI, category, and the healthy weight range for your height.
Frequently asked questions
What is a healthy BMI?
18.5 to 24.9 for most adults, per WHO guidance. The calculator also shows the weight range that corresponds to it for your exact height.
Is BMI accurate for everyone?
No — it cannot tell muscle from fat, so muscular people read high and older adults can read low. Treat it as a starting point, not a verdict.
How is BMI calculated?
Weight in kilograms divided by height in metres squared. In imperial units: 703 × pounds ÷ inches².
Should I worry about a result outside the range?
A single number is not a diagnosis. If your BMI is persistently outside the healthy band, a doctor can look at the fuller picture — waist, diet, activity and history.
More health-adjacent maths: the age calculator and percentage calculator — all part of our free daily tools.
What BMI measures, and what it does not
BMI is weight divided by height squared. That is the whole calculation, and it says nothing about what the weight is made of.
It was devised in the 1830s as a way of describing populations, not individuals, and it remains genuinely useful for that — tracking trends across a country, comparing groups. Applied to one person it is a rough screening number, which is how health services describe it and how it should be read.
Where BMI is straightforwardly wrong
- Muscular people. Muscle is denser than fat, so athletes routinely score “overweight” or “obese” while carrying very little fat.
- Older adults. Muscle mass falls with age, so a normal BMI can hide a high fat percentage.
- Children and teenagers. Adult categories do not apply; growth charts and percentiles are used instead.
- Pregnancy. The number is not meaningful.
- Ethnic variation. Health risk begins at lower BMI values in South Asian populations, and several health services use adjusted thresholds as a result.
Waist measurement often tells you more
Where fat sits matters more than how much there is. Fat around the abdomen carries markedly higher metabolic risk than the same weight on hips and thighs, and BMI cannot see the difference.
Waist circumference, or waist-to-height ratio, captures it in one measurement you can take with a tape measure. The commonly used guide is keeping your waist under half your height.
Reading the number sensibly
The standard adult categories are: under 18.5 underweight, 18.5–24.9 healthy, 25–29.9 overweight, 30 and above obese. Those boundaries are population thresholds, not cliffs — nothing changes about your health between 24.9 and 25.0.
Treat the number as one input among several: waist measurement, blood pressure, resting heart rate, how you actually feel. If it prompts a conversation with a doctor, it has done its job. It is not a diagnosis, and no calculator on the internet can give you one.
Calculated on your device
Your height and weight are never transmitted — the calculation runs in your browser and nothing is stored. For unit conversion between metric and imperial see the unit converter, and the percentage calculator for working out changes over time.